This section provides background information related to the present disclosure, which is not necessarily prior art.
Heat pumps can be useful in a variety of applications. For example, heat pumps can be used in plug-in hybrid electric vehicles (PHEV) and electric vehicles (EV) for heating the vehicle cabin. Heat pumps are particularly useful because they are more efficient than other heating systems, assemblies, and devices, such as electric heaters. Energy saved with a heat pump can be used to extend the driving range of the vehicle using the onboard battery in electric mode.
In PHEV applications that include an engine, it can be desirable to have the option of using either heat from the heat pump or heat from the engine when the engine is running in order to heat the cabin. To provide this option, a water or coolant circuit for heating the cabin with a heater core can be used. In a coolant circuit, hot coolant from the engine is used to heat the heater core when the engine is running. When the engine is not running, the coolant is heated by an electric heater or a heat pump system.
When using an electric heater, the heater can be switched on or off to regulate the temperature of coolant flowing to the heater core. The target coolant temperature is typically about 80° C. This is true in cold conditions where maximum heating is called for, as well as in mild conditions in which warm coolant is needed to re-heat the HVAC to provide occupants with a comfortable air temperature.
When the heat pump is in full heating mode, compressor speed can be adjusted to maintain the coolant at a temperature of about 80° C. However, in cooling mode, particularly in a closed loop cooling mode in which coolant does not pass through the engine, refrigerant exiting the compressor can be as high as 130° C., which can damage components of the water circuit. In an HVAC system in which heat is generated by an electric heater, the electric heater can simply be turned off to let the coolant temperature cool if it gets too hot.
But with a heat pump, if the compressor speed is reduced to prevent the coolant from becoming too warm, air conditioning performance may suffer and the temperature of the vehicle cabin may become undesirably warm. If cooler coolant from the engine is allowed to flow into the heat pump to cool coolant in the heat pump, temperature change experienced in the condenser may be too drastic and may undesirably affect the refrigerant cycle. For example, refrigerant may start to condense in the condenser, which happens in heating mode, thus adding liquid refrigerant to the condenser. The line extending between the water-cooled condenser and an outside heat exchanger, as well as the entire outside heat exchanger, may be undesirably filled with liquid refrigerant, which can result in a lack of refrigerant in the rest of the system for refrigerant starvation in the rest of the system).
A system for regulating coolant temperature of a heat pump system in cooling mode to allow for optimal operation of the heat pump, and to reduce or eliminate condenser condensation, would thus be desirable.